I am planning to get a new motherboard and processor to put in a 250D, because i have a CX750 non modular PSU, i'm wondering if i can safely tuck the wires where it won't affect airflow, as i can't replace my CX750 for a modular one. In addition, will an EVGA GTX 970 FTW+ fit in the case without interfering with the optical drive bracket?
Thankyou.

Powersupply, I would seriously consider getting at least a semi modular unit, I have an AX760i with only the 24 pin, 8 pin cpu, 2x Pcie power and 1x sata power connected and it's full. I'll try and upload a picture later, I had to resort to physically shortening the cables myself to try and save space.

That is a lot of wiring. For cooling, i use the stock fan, saving up money for a better cooler, though there's too many options, i was thinking be quiet pure rock. I hear noctua is good but it's not sold here. Water cooling is out of the equation as it's too expensive.

It is a lot of wiring and gets worse with a non-modular PSU :(
You could tuck everything down low, but with some cables like the 24-pin power, 8-pin power and SATA cables, they all make a mess unless you mod them.

You will have to watch with most tower units when installing in this case as the total height for a CPU cooler is only 95mm. The Be Quiet Pure Rock CPU cooler is 155mm or example.

AIO WC units can be as expensive as you want them to be. I don't know where you live, but a shop near me sells Corsair H60's for £35 on offer and you can pick up some bargains in the second-hand market.

I forgot about the height limitation, and since you're asking, i live in cyprus, there are no second hand markets for computer parts, and there's like 1-2 shops that sell parts (most "tech shops" sell pre-built computers, smartphones and tables) so they bump up the price quite a bit, on amazon i found the h60 and h100i for 60 and 96 pounds respectively, which is 80 and 130 euros, the shops here sell them for 110 and 170 euros respectively, which is quite frankly absurd.